cksum/cksum.1

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  35 .\"     @(#)cksum.1     8.2 (Berkeley) 4/28/95
  36 .\" $FreeBSD: src/usr.bin/cksum/cksum.1,v 1.19 2005/01/17 07:44:13 ru Exp $
  37 .\"
  38 .Dd April 28, 1995
  39 .Dt CKSUM 1
  40 .Os
  41 .Sh NAME
  42 .Nm cksum ,
  43 .Nm sum
  44 .Nd display file checksums and block counts
  45 .Sh SYNOPSIS
  46 .Nm
  47 .Op Fl o Ar 1 | 2 | 3
  48 .Op Ar
  49 .Nm sum
  50 .Op Ar
  51 .Sh DESCRIPTION
  52 The
  53 .Nm
  54 utility writes to the standard output three whitespace separated
  55 fields for each input file.
  56 These fields are a checksum
  57 .Tn CRC ,
  58 the total number of octets in the file and the file name.
  59 If no file name is specified, the standard input is used and no file name
  60 is written.
  61 .Pp
  62 The
  63 .Nm sum
  64 utility is identical to the
  65 .Nm
  66 utility, except that it defaults to using historic algorithm 1, as
  67 described below.
  68 It is provided for compatibility only.
  69 .Pp
  70 The options are as follows:
  71 .Bl -tag -width indent
  72 .It Fl o
  73 Use historic algorithms instead of the (superior) default one.
  74 .Pp
  75 Algorithm 1 is the algorithm used by historic
  76 .Bx
  77 systems as the
  78 .Xr sum 1
  79 algorithm and by historic
  80 .At V
  81 systems as the
  82 .Xr sum 1
  83 algorithm when using the
  84 .Fl r
  85 option.
  86 This is a 16-bit checksum, with a right rotation before each addition;
  87 overflow is discarded.
  88 .Pp
  89 Algorithm 2 is the algorithm used by historic
  90 .At V
  91 systems as the
  92 default
  93 .Xr sum 1
  94 algorithm.
  95 This is a 32-bit checksum, and is defined as follows:
  96 .Bd -unfilled -offset indent
  97 s = sum of all bytes;
  98 r = s % 2^16 + (s % 2^32) / 2^16;
  99 cksum = (r % 2^16) + r / 2^16;
 100 .Ed
 101 .Pp
 102 Algorithm 3 is what is commonly called the
 103 .Ql 32bit CRC
 104 algorithm.
 105 This is a 32-bit checksum.
 106 .Pp
 107 Both algorithm 1 and 2 write to the standard output the same fields as
 108 the default algorithm except that the size of the file in bytes is
 109 replaced with the size of the file in blocks.
 110 For historic reasons, the block size is 1024 for algorithm 1 and 512
 111 for algorithm 2.
 112 Partial blocks are rounded up.
 113 .El
 114 .Pp
 115 The default
 116 .Tn CRC
 117 used is based on the polynomial used for
 118 .Tn CRC
 119 error checking
 120 in the networking standard
 121 .St -iso8802-3 .
 122 The
 123 .Tn CRC
 124 checksum encoding is defined by the generating polynomial:
 125 .Pp
 126 .Bd -unfilled -offset indent
 127 G(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 +
 128      x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
 129 .Ed
 130 .Pp
 131 Mathematically, the
 132 .Tn CRC
 133 value corresponding to a given file is defined by
 134 the following procedure:
 135 .Bd -ragged -offset indent
 136 The
 137 .Ar n
 138 bits to be evaluated are considered to be the coefficients of a mod 2
 139 polynomial M(x) of degree
 140 .Ar n Ns \-1 .
 141 These
 142 .Ar n
 143 bits are the bits from the file, with the most significant bit being the most
 144 significant bit of the first octet of the file and the last bit being the least
 145 significant bit of the last octet, padded with zero bits (if necessary) to
 146 achieve an integral number of octets, followed by one or more octets
 147 representing the length of the file as a binary value, least significant octet
 148 first.
 149 The smallest number of octets capable of representing this integer are used.
 150 .Pp
 151 M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided by
 152 G(x) using mod 2 division, producing a remainder R(x) of degree <= 31.
 153 .Pp
 154 The coefficients of R(x) are considered to be a 32-bit sequence.
 155 .Pp
 156 The bit sequence is complemented and the result is the CRC.
 157 .Ed
 158 .Sh EXIT STATUS
 159 .Ex -std cksum sum
 160 .Sh SEE ALSO
 161 .Xr md5 1
 162 .Pp
 163 The default calculation is identical to that given in pseudo-code
 164 in the following
 165 .Tn ACM
 166 article.
 167 .Rs
 168 .%T "Computation of Cyclic Redundancy Checks Via Table Lookup"
 169 .%A Dilip V. Sarwate
 170 .%J "Communications of the" Tn ACM
 171 .%D "August 1988"
 172 .Re
 173 .Sh STANDARDS
 174 The
 175 .Nm
 176 utility is expected to conform to
 177 .St -p1003.2-92 .
 178 .Sh HISTORY
 179 The
 180 .Nm
 181 utility appeared in
 182 .Bx 4.4 .